Any complex adaptation, requiring many genes to work together, cannot all evolve at once, it would be too unlikely a mutation. Instead, pieces evolve one by one, each individually useful in the context they first appear. However, there is not enough selection pressure to evolve a new piece unless the old pieces are already universal, so you would not expect anything complicated to exist in some but not all members of a species.

With intelligence, it seems like many different factors can affect it on the margins, because the brain is a complex organ that can be slowed down, sped up or damaged in many ways. However, I do not notice a particularly wide intelligence spread among humans, only in rare cases where something is genuinely broken do we find someone less intelligent than a chimpanzee, and we literally never find someone more intelligent by an equivalent amount.